Conventionally, known classifying apparatuses include rotary mechanical classifying apparatuses for dividing micron-order powder material into coarse powder and fine powder. The rotary mechanical classifying apparatuses have a mechanism in which the powder material is centrifugally classified utilizing the centrifugal force of a rotating rotor. Each rotary mechanical classifying apparatus includes a rotor with a plurality of annularly disposed blades, which rotates; a louver with a plurality of blades which are disposed at an outer circumferential portion of the rotor so as to supply from the outer circumferential portion a fluid for dispersing and classifying a powder material; and a classification chamber having a shape based upon the cylindrical shape of the classifying apparatus, where the powder material supplied to a gap between the rotor and the louver is centrifugally classified into fine powder and coarse powder. Affected by the flow sucked from inside the rotating rotor with the plurality of annularly disposed blades and the flow of the rotating rotor, the powder material supplied to the classification chamber, which is the gap between the rotor and the louver, is divided into powder material led toward the inside of the rotor and powder material led toward the outside of the rotor, depending upon the balance of the force applied to powder particles, then discharged to a fine/coarse powder outlet, and thus classified into coarse powder and fine powder.
As to a conventional rotary mechanical classifying apparatus, the louver (6) in FIG. 1, described later, is not a specific louver employed in the present invention.
Continuous classification is possible in such a conventional rotary mechanical classifying apparatus. However, to separate powder of a desired particle diameter in the conventional rotary mechanical classifying apparatus, it is necessary to always provide the powder material with a uniform force balance in the classification chamber. In reality, though, it is difficult to provide each piece of the powder material with a uniform force balance, which is a cause of reduction in classification efficiency.
Examples of classifying apparatuses with improved classification efficiency include the rotary mechanical classifying apparatus disclosed in PTL 1. In this classifying apparatus, a classifying rotor in the form of an impeller is rotated, for example, around an axial core with respect to the perpendicular direction so as to swirl raw material powder, and classification air is blown from a classification space formed on the outer circumferential side of the classifying rotor toward the inside of the classifying rotor with respect to the radius direction, whereby fine powder in the raw material powder is carried by the flow of the classification air and passes through classifying blades because it is affected more by the conveying force of the airflow than by the centrifugal force related to the rotation, while coarse powder is thrust toward the outside and so cannot pass through the classifying blades because it is affected more by the centrifugal force related to the rotation, and thus the raw material powder is classified into fine powder and coarse powder.
Meanwhile, regarding the rotary mechanical classifying apparatus disclosed in PTL 2, there is proposed a classifier for toner, wherein the classifying rotor in the form of an impeller is coaxially placed in two places in one casing, raw material powder is sequentially passed through classification spaces on the outer circumferential side of the two classifying rotors, fine powder discharged from a fine powder outlet of the classifying rotor on the anterior side is designated as fine powder, fine powder discharged from a fine powder outlet of the classifying rotor on the posterior side is designated as middle-sized powder, coarse powder discharged from a coarse powder outlet is designated as coarse powder, thereby classifying the raw material powder such as toner into the three sections of fine powder, middle-sized powder and coarse powder, and the middle-sized powder is employed as a toner product.
However, regarding these classifying apparatuses, after the classification, coarse powder may be present among fine powder, or fine powder may be present among coarse powder, which leads to a decrease in classification accuracy and product yield, so that it is necessary to reduce the presence as much as possible.
In the rotary mechanical classifying apparatus disclosed in PTL 3, classifying blades of a classifying rotor are formed such that an outer end thereof with respect to the rotor diameter direction protrudes toward the outside with respect to the rotor diameter direction to a greater extent at the center than both ends thereof with respect to the rotor shaft direction. Regarding this structure, the outer end of the classifying blades, positioned at the center with respect to the rotor shaft direction, protrudes toward the outside with respect to the rotor diameter direction to a greater extent than the outer ends of the classifying blades, positioned at both ends with respect to the shaft direction; therefore, the circumferential speed becomes even higher, and thus the centrifugal force related to the rotation of the classifying rotor increases.
Specifically, an increase in the force of conveying coarse powder, which is due to the increase in airflow speed at the center with respect to the rotor shaft direction, is nullified by the increase in centrifugal force, so that coarse powder is prevented from thrusting from the outer circumferential side of the classifying rotor into the rotor, and thus the presence of coarse powder among fine powder discharged by suction from inside the rotor is restrained. It is alleged that a classifier is provided which is capable of restraining the presence of coarse powder among fine powder and thus improving classification accuracy and product yield; for example, in the case where fine powder is a product, the presence of coarse powder among fine powder is restrained so as to increase classification accuracy for fine powder; in the case where coarse powder is a product, the presence of coarse powder among fine powder is restrained so as to increase the product yield of coarse powder.
However, even when the classifying blades in the classifying apparatus are formed in the above-mentioned manner, only part of the centrifugal force slightly increases, which is insufficient for improvement in classification accuracy and product yield. This is because, in the classifier with the above-mentioned structure, fine powder is sucked together with air by a suction device or the like from inside the rotating classifying rotor, being sucked from one side inside the rotor, and thus there is a distribution of the speed of the air inside the rotor and the air sucked into the rotor from the classifying blades. Specifically, there is a distribution with respect to the width direction (the vertical direction in the case where the rotor is a longitudinally placed rotor) of the classifying blades, the effects thereof being uncertain. In other words, when powder material is classified utilizing the centrifugal force of air, what is important is a balance of the force applied from the air to the powder material; in the case of this classifier, the powder material is classified into coarse powder and fine powder according to the balance between the centrifugal force generated by the rotation of the rotor and the force of suction from inside the rotor. Therefore, regarding the structure disclosed in PTL 3, the balance between the powder material and the suction air is not considered, and this may lead to a further decrease in classification accuracy and product yield.
In PTL 3, as a fourth characteristic, provision of such a suitable embodiment of a classifier as follows is described: an annular classification space is formed between an outer circumferential surface of a classifying rotor and a guide blade ring with a plurality of annularly disposed guide blades, with a certain amount of space existing between the guide blades and the outer circumferential surface, a classification air supply unit for supplying classification air to the classification space through gaps between the adjacent guide blades of the guide blade ring is provided, raw material powder brought into a swirling state by the rotation of the classifying rotor reaches the annular classification space situated between the outer circumferential surface of the classifying rotor and the surrounding guide blade ring, fine powder is conveyed from the outer circumferential surface of the classifying rotor into the rotor by classification air supplied through the gaps between the adjacent blades of the guide blade ring, coarse powder is thrust away by classifying blades and thus not conveyed into the rotor but passes through the classification space, and it is thereby possible to increase classification accuracy and product yield further.
However, depending upon the structure of the above-mentioned guide blade ring, the distribution of speed related to centrifugal force may not be uniform in the space formed between the guide blade ring and the outer circumferential surface of the classifying rotor, coarse powder may not be thrust away by the classifying blades but conveyed into the rotor, which leads to a decrease in classification accuracy and product yield, and so there is room for improvement in the structure.